The present invention relates to an electrophotographic copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention relates to a fixing device for fixing a toner image on a recording medium by conveying the medium via a nip where a fix roller over which the belt is passed and a press roller are pressed against each other.
It is a common practice with an electrophotographic image forming apparatus to cause a scanner to read a document image and then and cause an image forming section to electrostatically form a latent image representative of the document image on a photoconductive element or similar image carrier by exposure. The latent image is developed to form a corresponding toner image. The toner image is transferred from the drum to a paper or similar recording medium and then fixed on the paper. The paper with the fixed toner image is driven out of the apparatus.
A predominant type of fixing device for the above application uses a heat roller having a heat source thereinside and a press roller pressed against the heat roller. While the heat roller and press roller are rotated while forming a nip therebetween, they fix a toner image formed on a paper being conveyed via the nip by heat and pressure. The prerequisite with this type of fixing device is that an elastic layer covering the surface of the fix roller be thin enough to prevent a warm-up time from increasing. However, the limited thickness of the elastic layer limits the available width of the nip and therefore a parting temperature range insuring desirable fixation. The parting temperature range refers to a range between a cold offset condition and a hot offset condition. In the cold offset condition, the fixing temperature is so low, toner adheres to the fix roller and belt or cannot be sufficiently fixed on the paper. In the hot offset condition, the fixing temperature is so high, adhesion acting between toner particles is weakened and causes the toner to again adhere to the fix roller and belt.
Japanese Patent Laid-Open Publication No. 8-339133, for example, teaches a fixing device using a belt and locating a heat source outside of a fix roller in order to reduce the warm-up time. Specifically, the belt has a preselected area and is heated in order to scatter the time and place for heating toner carried on a recording medium. With this scheme, it is possible to select a relatively low fixing temperature and therefore to reduce the warmup time as well as required energy. The fixing device taught in the above document is also usable as means or a device of the type heating a recording medium, e.g., a device installed in a copier, laser beam printer (LBP), electrostatic printer or similar image forming apparatus for fixing a toner image on a recording medium by heat and pressure or a device for heating a recording medium carrying an image in order to regenerate its surface property.
However, the above conventional fixing device has some problems left unsolved, as will be described specifically later.